The occurrence and growth of Listeria monocytogenes on rainbow trout fillets packaged in different atmospheres

In the first part of this study rainbow trout samples purchased at retail markets were surveyed for the presence of Listeria. Samples were both direct plated and enriched following the USDA and FDA protocols. Plating was performed on PALCAM and MOX agar directly and after 24 and 48 h enrichments. A total of 40 samples (54.1 %) tested positive for Listeria in one of the plating periods. Eight positive samples (20%) were detected by direct plating on MOX agar; seven (17.5%) were detected on PALCAM. Thirty-nine of 40 positive samples (97.5%) were detected by the FDA procedure, while the USDA procedure detected 33 positive samples (82.5%). Neither procedure detected all of the positive samples at either time period. PALCAM was more selective and resulted in more isolated colonies after 24 h enrichment, but was not as effective as MOX for direct plating. From these data, it was concluded that the FDA procedure is superior to the USDA procedure for isolation of Listeria from fresh trout; however, the best results were obtained by a combination of enrichment and plating media.

The second part of this study examined the growth of Listeria monocytogenes on rainbow trout fillets, packaged in air, vacuum, and a 60% CO₂/40% N₂ modified atmosphere (MA) at 3 or 10°C. Fillets showed signs of spoilage (off odor) when the aerobic plate count reached approximately log 7 CFU/g. At 10°, the aerobic plate count reached log 7 CFU/g after 2 days for the fillets packaged in air and after 4 days for the fillets packaged in vacuum and the MA. At 3°C, aerobic plate count reached log 7 CFU/g after 9 days for the fillets packaged in air and vacuum and after 15 days for the fillets packaged in the MA. Listeria was inoculated onto the surface of the fillets at approximately log 3 CFU/g; however only log 2 CFU/g was detectable initially. After 2 days at 10°C, Listeria numbers increased to log 4 CFU/g in the air packaged fillets; whereas in the vacuum and MA after 4 days, Listeria numbers on the fillets increased to log 4.5 CFU/g. After 9 days at 3°C, Listeria numbers increased to log 3 and 4 for the fillets packaged in vacuum and air, respectively. After 15 days at 3°C, Listeria numbers also increased to log 3 CFU/g in the fillets packaged in the MA. These data indicate that, although a high CO₂ atmosphere can increase the shelf-life of rainbow trout fillets by inhibited the growth of the spoilage bacteria and improve the safety of the product by inhibiting the growth of Listeria, strict temperature control has a greater effect and should be the primary concern.